Infants' motor skill development triggers changes in parent-infant interactions, exploration, and play behaviors, particularly during periods of locomotor transitions. We investigated how these transitions reorganized infants' and mothers' explorations of spatial layouts. Thirteen infants and their mothers were followed biweekly from the age of 6 to 17 months. This report focused on 2 periods of 6 sessions surrounding infants' hands-and-knees crawling and walking onsets. Infants' and mothers' activities were monitored during 10-min free-play sessions held in a laboratory room provided with toys and furniture. Using location coordinates for the mother and infant, we derived several measures of spatial displacement and exploration as both mother and infant moved around the room. We also observed variations in mothers' and infants' interactive behaviors and postural changes within and across sessions. Infants increased interactive behaviors, traveled further distances, and visited more places in the room over time than their mothers. This increase occurred particularly after infants became experienced hands-and-knees crawlers. The distance between infant and mother and number of postural changes also increased as infants became more mobile. This study reveals that mother-infant explorations of spatial layouts diversify and reorganize over time as infants develop new locomotor skills.